As mammals age their lipids undergo various types of changes in composition in different organs. For example in heart linoleate, 18:2 (n-6), is partly replaced by 22:5 (n-3) and 22:6 (n-3) in rats 24 months old. In liver, arachidonate, 20:4 (n-6), also contributes to the loss of essential fatty acids. The major fatty acid of testis, 22:5 (n-6), declines with age; it too is replaced in part by 22:5 (n-3) and 22:6 (n-3). These and other changes occur while the animal is on a diet rich in essential fatty acids. It is proposed to measure rates of fatty acid synthesis by perfused organs and in vivo and the rates at which the newly synthesized fatty acids go into the various classes of lipids. The reasons why certain organs fail to produce or accumulate their specific fatty acids as the animal becomes old will be investigated. Rates of chain elongation and desaturation of members of the linoleate and linolenate family will be measured and related to the activities of the enzymes concerned. The objective of this proposal is to characterize the changes in composition and end-product distribution of fatty acids that occur in various organs with age in terms of changes in enzyme activities, and to understand the underlying regulatory factors. Another objective is to test whether the changes in composition of the (n-6) and (n-3) families of fatty acids give rise to changes in prostaglandin production. It is hoped that the research project will increase our understanding of the molecular and cellular basis of aging.